In the telecommunications field, contention is a problematic condition that arises when two or more devices attempt to use the same network resource at the same time. For example, contention arises when two network devices attempt to transmit over a shared channel at the same time. Contention resolution is the process of deciding which device can access the network resource first.
In existing telecommunication networks, contention resolution is accomplished in numerous ways. For example, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is a shared-medium contention resolution scheme typically used in Ethernet networks. Basically, with CSMA/CD, as a user (device) is transmitting a packet, that user also measures the energy level of the transmission medium at the same time. If any user determines that one or more other user(s) is also transmitting packets, all of the users terminate their transmissions, and wait a random amount of time (known as back-off delay) before attempting to transmit again.
CSMA with Collision Avoidance (CSMA/CA) is another shared-medium contention resolution scheme typically used in wireless communication networks (e.g., WLANs, Wi-Fi, etc.). With CSMA/CA, when a user (device) wants to transmit a packet, that user first has to listen to the channel for a predetermined amount of time, in order to check for any activity on the channel. If the user determines that the channel is idle, then that user is allowed to transmit. If the user determines that the channel is busy (a second user is transmitting a packet), then the first user has to defer its transmission.
Notwithstanding the numerous advantages of the CSMA/CD and CSMA/CA contention resolution schemes, there are numerous contention situations where these schemes are not applicable. For example, both CSMA/CD and CSMA/CA are not applicable if the devices in contention for the same network resources are incapable of hearing each other.
A typical situation that arises, in which CSMA/CD and CSMA/CA are not applicable contention resolution schemes, is where two (or more) Base Stations (BSs) are contending for spectrum ownership, and each BS is located outside of the other's radio coverage area. In this case, the contending BSs are unable to hear each other, and CSMA/CA for contention resolution is not a viable approach. Also, in this case, the contending BSs are not located within one Ethernet network, so CSMA/CD for contention resolution is also not a viable approach. Notably, for Cognitive Radio-based (CR-based) network designs (IEEE 802.22 standard) with dynamic frequency selection, this situation raises a legitimate concern.
Specifically, in CR-based networks using dynamic frequency selection, each BS is designed to cover a distance that corresponds to the radius of one cell (i.e., not the double-radius distance between two neighbor BSs). Consequently, in CR-based networks with dynamic frequency selection, the ability to establish direct wireless links between neighbor BSs is not guaranteed. Also, in CR-based networks with dynamic frequency selection, the use of CSMA/CD for contention resolution is not a viable approach. For example, availability and reliability design requirements impose limitations on carrier sensing distances in the Ethernet networks involved. These limits are generally much smaller than the direct distances between neighbor BSs. As a practical matter, these limits are even smaller due to the signal propagation effects of various terrains.
In this regard, FIG. 1 is a diagram of an example CR-based network 100, which illustrates a related problem of determining how to resolve contention if a wireless or wireline network resource is not to be directly shared. Referring to FIG. 1, assume that there is only one channel (e.g., television channel) available at a certain time, and a time-division channel sharing scheme is to be used. However, if the BSs 102, 104 and 106 in network 100 are unable to communicate and coordinate with each other directly over the air, then the CPEs 108-118 in network 100 will experience inter-cell interference, and the time-division channel sharing scheme will fail. Consequently, the BSs 102, 104, 106 will have to be able to communicate and coordinate with each other in some way, in order to resolve the time-division channel sharing contention issues involved. Therefore, there is a pressing need for a contention resolution technique that can be used in wireless or wireline telecommunication networks if the devices in contention are unable to communicate with each other, and CSMA/CD and CSMS/CA are also not applicable contention resolution techniques.
Additionally, inefficiency is a contention-related problem that arises when two or more devices use the same network resource at the same time. For example, in conventional wireless communication networks, the BSs typically assign frequency channels to users on an as-needed basis from dedicated radio spectrum purchased by the network operators. However, there is a substantial amount of unused radio spectrum that could be used, if the networks could recognize that the channels are available and then assign the unused channels to the users involved. A practical illustration of this problem involves the use of idle television channels.
The IEEE 802.22 Working Group on Wireless Regional Area Networks (WRANs) is developing the specifications for a fixed point-to-multipoint WRAN that will utilize specific television channels and guard bands for communications in the UHF and VHF television bands. A primary goal of the Working Group is to develop the standard for a CR-based Physical Layer/Medium Access Control (PHY/MAC) air interface for use by license-exempt wireless communication devices on a non-interfering basis in a frequency spectrum allocated for broadcast television. Essentially, the broadcast television spectrum is divided up into numerous channels, but many of these channels remain unused for long periods of time. However, using CR-based techniques, it is now possible to deploy a WRAN that can allocate portions of the unused television spectrum to Customer Premises Equipment (CPE) for use during a set period of time. The WRAN can allocate these unused television channels in such a way that broadcasts on these channels will not interfere with broadcasts by the stations that normally use the television spectrum.
In certain CR-based network configurations, a WRAN may include a plurality of BSs, and each BS in the WRAN has a radio coverage area that may include several cells and sectors. Thus, the WRAN can be designed to cover an entire metropolitan area with a network of BSs. However, notwithstanding the advantages of such a network configuration, a number of significant contention-related problems exist in this regard.
For example, the radio coverage areas of numerous BSs in the WRAN may overlap. The CPEs located within the radio coverage area of a BS are able to determine the strength of the signal received from that BS, by searching for a pilot signal being broadcast by that BS on an idle television channel. Consequently, a CPE located in a region covered by multiple BSs may see several pilot signals at once. As another example, two or more CPEs may be located in close proximity to each other, but each such CPE may be served by a different BS.
Notably, in CR-based network designs, contention issues arise whenever two BSs are allowed to use the same unused television channel. Consequently, it would be advantageous if the BSs were able to share the channel between them. However, in order to share a channel, the BSs have to be synchronized, and they also have to be able to communicate with each other with respect to scheduling the channel for their respective CPEs. In conventional networks, these functions are accomplished for dedicated spectrum deployments, by using network signaling over the air between the BSs involved. However, in WRAN deployments using CR-based methods to sense the unused spectrum, the conventional techniques will require the CPEs to inform each other and the BSs about their channel assignments. Unfortunately, this approach will use up a substantial amount of the valuable television broadcast spectrum for these signaling functions.
Furthermore, in order to avoid interference, it would be advantageous if the BSs did not assign the same channel to the CPEs they are servicing. Thus, the BSs are able to avoid interference if they can keep each other informed about their spectrum allocations. In conventional networks, these functions are accomplished for dedicated spectrum deployments, by using network signaling over the air between the BSs involved, or using pre-assigned frequencies for each BS. However, in WRAN deployments, the conventional techniques will require the CPEs to inform each other and the BSs about their channel assignments. Unfortunately, this approach will use up a substantial amount of the valuable television broadcast spectrum for these signaling functions.